Dual tone multi-frequency (DTMF) signals normally consist of two simultaneous tones for designating a dialed digit, one from a group of high frequency tones, and the other from a group of low frequency tones. The four DTMF tones whose nominal frequencies are 697, 770, 852 and 941 Hertz comprise the low group tones, while the four DTMF tones whose nominal frequencies are 1209, 1336, 1477 and 1633 Hertz comprise the high group tones.
Prior art analog tone receivers are well known for decoding DTMF tones in pairs. Such prior art analog receivers are typically of complex and expensive design, and have been found to yield inaccurate results. Furthermore, as a result of the trend towards digitization of PABXs and telephone central offices, many prior art analog tone receivers are quickly becoming obsolete.
In an effort to overcome the disadvantages of prior art analog tone receivers, and in keeping with the aforementioned trend towards digitization, a number of digital tone receiver circuits have been developed.
One such circuit is described in U.K. patent GB 2,049,360 (Ikeda), wherein an input signal sample is convolved with sampled values of reference signals having predetermined frequencies corresponding to the frequencies to be detected. The convolution is in the form of a discrete Fourier transform (DFT) which yields two series of trigonometric product values from which the spectrum components of the input signal can be determined at the desired frequencies.
A further prior art digital tone receiver is described in an article entitled "Add DTMF Generation and Decoding to DSP- .mu.P Designs", by patrick Mock, published by Electronic Design News, Mar. 21, 1985. According to this latter prior art digital tone receiver, a discrete Fourier transform (DFT) is implemented according to what is known in the art as Goertzel's algorithm. The main advantage of using Goertzel's algorithm over the DFT approach used in the aforementioned U.K. patent, is that only one real coefficient is required to be generated per detection frequency in order to determine the magnitude of the signal component at the detection frequency.
Both prior art DFT based digital tone receivers suffer from the disadvantage that in order to obtain a sufficiently accurate measurement of the incoming signal frequency, a very lengthy and complex DFT is required to be calculated, resulting in very slow detection speed. Conversely, in the event that a fast and simple DFT is implemented, the detected tone cannot typically be ascertained with a sufficient degree of accuracy to comply with national and industry standard specifications for DTMF tone detection.
One approach to overcoming this two-fold prior art disadvantage, has been to execute two successive fast DFT detection algorithms on an incoming signal, at a low level of accuracy. If the results of both DFT detection algorithms indicate that a DTMF tone has been detected, then the tone is indicated as being present.
This approach has been found in general to be deficient since the level of tone detection talk-off (simulation of DTMF tones by speech), or other causes of erroneous tone detection.